1. Field of the Invention
The present invention relates to a switching power supply, and more particularly to a switching power supply that can be started up reliably and can operate at a reduced power consumption level during the steady operation.
2. Description of the Related Art
FIG. 8 is a block diagram illustrating a conventional switching power supply 801. In the switching power supply 801, an AC voltage input from the outside is converted to a DC voltage through a rectifier circuit 808 and a smoothing capacitor C801, and then converted, at a switching circuit 802, to an AC voltage of a higher frequency than the frequency of input AC voltage. Then, the AC voltage is output from the switching circuit 802 to the outside via a transformer 803 including a primary coil 809 and a secondary coil 810. Moreover, in the transformer 803, a voltage is obtained from a tertiary coil 811, which is used for driving a control circuit 813. Specifically, a constant voltage circuit 812 receives the voltage from the tertiary coil 811 and provides a constant voltage to the control circuit 813. The control circuit 813 is driven by the constant voltage provided thereto to turn ON/OFF the switching circuit 802. A start-up circuit 805 provides a power supply voltage V1 to the constant voltage circuit 812 via a resistor R801, whereby the control circuit 813 can be driven at the start-up of the switching power supply. Otherwise, the switching power supply may not be started up successfully because, at the start-up of the switching power supply, the switching circuit 802 is not operating, and the voltage from the tertiary coil 811 for driving the control circuit 813 is not available.
In the conventional switching power supply 801, a voltage is occurred at the resistor R801 due to the difference between the power supply voltage V1 and a voltage V2 from the tertiary coil 811, whereby a current constantly flows to a capacitor C802, not only at the start-up but also during the steady operation. This results in a wasteful power consumption at the resistor R801 during the steady operation.
In order to solve this problem, a switching power supply 901 as illustrated in FIG. 9 (only a start-up circuit 905 is shown for the sake of simplicity) has been proposed in the art. As an external power supply is input to the switching power supply 901 at the start-up, a current flows into the capacitor C802 and constant voltage circuit 812 via a resistor R901 and a capacitor C903 while in the transient state, and the level of the current decreases with a predetermined time constant. Then, during the steady operation, a current that is determined by the resistance of the resistor R901 and that of a resistor R902 flows into the capacitor C802. Therefore, the transient current at the start-up can be increased by decreasing the resistance of the resistor R901, and the current flowing during the steady operation can be decreased by increasing the resistance of the resistor R902, whereby it is possible to reduce the wasteful power consumption. Moreover, the amount of time for which the transient current flows can be controlled by adjusting the value of the capacitor C903.
With the switching power supply 901, however, the amount of transient current flowing into the capacitor C802 and constant voltage circuit 812 varies substantially depending on the residual voltage of the voltage V1 and the residual charge stored in the capacitor C903 at the time of power-on, whereby the switching power supply 901 may not be started up reliably.
Furthermore, Japanese Laid-Open Patent Publication No. 9-93918 describes switching power supply for turning off the switching means 3, upon detection of a vibration voltage generated in an output of the tertiary coil 63 of the transformer 6 at the time of starting the intermittent operation of the power supply control circuit 11. However, in the switching power supply, a resistor element 42 is indispensable because of turning OFF MOSFET 31 with voltage drop of the resistor element 42. Therefore, because the power supply control circuit 11 is always driven by a current flow via the resistor element 42, a wasteful power consumption is caused at the resistor element 42 during the steady operation.